Color digital imaging systems, such as digital cameras, typically employ image sensors, such as charge coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) devices to digitally capture a scene of interest. Image sensors typically include an array of optical detectors, such as photodiodes, that generate an electrical response in proportion to the intensity of incident light. The dynamic range of individual optical detectors is defined by the minimum amount of light that is required to generate an electrical response at the low end and the maximum amount of light beyond which the electrical response of the optical detector does not change (i.e. a saturation point) at the high end.
There is a trend toward developing imaging systems to produce high dynamic range (HDR) images. Output devices (e.g., display devices or printers) for presenting HDR images, however, typically have a more limited dynamic range. Accordingly, there is a need for techniques to compress the dynamic range of an image to correspond to the dynamic range of the output device. One such technique for dynamic range compression includes tone mapping. Tone mapping, in general, maps captured scene luminance levels of an HDR image to the luminance or density levels of the output device.
Tone mapping is typically categorized as relating to global tone mapping techniques or local tone mapping techniques. Global tone mapping is generally based on a global luminance adaptation of the HDR image, whereas local tone mapping is based on a local luminance level adaptation of the HDR image. Accordingly, global tone mapping techniques are generally less computationally intensive than local tone mapping techniques. An objective of tone mapping (both global and local tone mapping) is to provide an output image with a lower dynamic range, while attempting to preserve the image detail and color appearance of the originally captured image. Because tone mapping reduces the dynamic range and, thus, the contrast of the HDR image, the compressed image may still appear flat and dull. The compressed image may also include a subjectively unpleasing loss of detail as compared with the original HDR image.